The present invention relates to a fuel cell power generation system combining a fuel cell and a fuel reformer. A fuel cell generation system combines, as is well known, a fuel cell and a fuel reformer and generates power by supplying both a fuel gas including a large amount of hydrogen obtained, for example, by stream reformation of a natural gas and air extracted from the atmosphere as an oxidizing agent to the fuel cell.
Development of a pressurized fuel cell power generation system, which is to be operated with a cell operating pressure enhanced to about 4.about.7 kg/cm.sup.2 G from about 0.5 kg/cm.sup.2 G in the prior art, has recently been continued with the objective of realizing improvement in the power generation efficiency of a fuel cell.
Such a pressurized fuel cell power generation system requires an air compressor for supplying pressurized air extracted from the atmosphere to the fuel cell, and also requires a large power source to drive this compressor. In order to improve efficiency of the power generation system, a system has already been developed, and put into practical use, which utilizes heat in the system and exhaust gas from the reformer combustion as a power source to drive a turbo compressor for pressurizing the reaction air.
A process flowchart of a fuel cell power generation system described previously as the prior art is shown in FIG. 3. FIG. 3 illustrates, among other things, a fuel cell 1, a reformer 2, a turbo-compressor 3, an extra combustion chamber 4, a steam separator 5 and a pump 6 which circulates cooling water through the fuel cell, and the steam separator 5.
In FIG. 3, the natural gas used as a raw material for reforming is steam-reformed by the reformer 2 and is then supplied, as fuel gas including a large amount of hydrogen, to a gas space in the anode side of the fuel cell 1. The reaction air, as the oxidating agent, is pressurized by a compressor 3a of the turbo-compressor 3 and is then supplied to a gas space in the cathode side of the fuel cell 1, thereby enabling fuel cell 1 to generate electrical power. The off fuel gas and off air exhausted from the fuel cell 1 are supplied to a burner 2a of the reformer 2. These gases are burned to provide the necessary reforming reaction heat. The steam required for the reforming reaction is obtained from the steam separator 5. Moreover, as a driving source of turbine 3b of the turbo-compressor 3, high temperature off air exhausted from the fuel cell and extra high temperature steam, obtained by heating excess steam from the steam separator 5, are added to the combustion exhaust gas of the pressurized combustion furnace of reformer 2. This combination of high temperature and high pressure fluids is further heated in the extra combustion chamber 4 and is used to drive the turbocompressor 3 by guiding the extra combustion chamber exhaust gas to a turbine 3b.
The power generation system can be started by the following procedures. The burner of reformer 2 is first fired in order to raise the furnace temperature and pressure up to a predetermined value. The turbo-compressor 3 is then started by air supplied from an air source provided separately or externally and power generation is started by supplying fuel gas and air to the fuel cell 1. Upon confirmation that the fuel cell operation voltage has reached its operating level, the power output of the fuel cell is coupled to the power system.
Fuel cell power generation systems of the prior art are subject to certain problems. Namely, the power generation system of the prior art provides input energy for the turbo compressor 3 by directing the exhaust gas from the combustion of the reformer 2 to the turbo compressor and forms a closed loop, as previously explained, by increasing the reaction air pressure to be supplied to the fuel cell 1 with the output of the compressor. Therefore, if an interrelated control is not carried out in such a manner as to keep an adequate balance in the flow rate, pressure, and temperature of fluids flowing into each section of the system, especially during start-up and stopping of the power generation system and when disturbing factors such as variation of load occur, the pressure distribution of the system as a whole is changed and imbalances in pressure levels within the fuel cell and surging of the turbo compressor result.
To avoid these effects, a prior art turbo-compressor has generally been operated at a constant pressure and flow rate in order to prevent surging of the turbo-compressor and pressure variations and imbalances within the fuel cell. However, if operation of the reformer 2 is controlled according to changes in load, the temperature and pressure of the combustion exhaust gas used as a power source for the turbo-compressor 3 also changes. Moreover, the volume of air in the system functions as a buffer for very fast changes of load and results in a reduction in the flow rate of exhaust gas to the turbo-compressor within a short period. Accordingly, if countermeasures are not taken, energy to drive the exhaust turbine 3b of the turbo-compressor 3 becomes insufficient when a load change occurs. Especially, in a partial load, the outlet pressure of the compressor 3b changes and causes the reaction air pressure being supplied to the fuel cell to be reduced. This results in pressure differences within the fuel cell which are likely to damage the fuel cell.
The insufficient source of power for the turbocompressor 3 during a partial load has been addressed by employing a means of raising the temperature of the combustion exhaust gas of the reformer by inputting extra steam and supplying a fuel such as natural gas to an extra combustion chamber 4, as is shown in FIG. 3. However, even though the fuel cell itself has a higher power generation efficiency in the partial load operation, the system mentioned above, which employs an extra combustion chamber 4 in order to compensate for insufficient power energy of the turbo-compressor 3, will result in a lowered power generation efficiency of the power generation system as a whole because of the energy consumed in operating the added chamber 4. Moreover, from the point of view of the operation of the power generation system, employment of a turbo-compressor requires another external or supplemental air source for starting the turbo-compressor, and the reaction air cannot be supplied to the fuel cell 1 until after the turbo-compressor 3 has started operation. Moreover, the turbo-compressor requires, in general, careful starting operation in order to prevent surging, thus it takes a considerable period until reaction air pressure is raised up to the rated pressure. As a result, it is considered a factor in lengthening the starting time of the power generation system and also in limiting the ability to respond to load variations.